The present invention relates to a hydrothermal gel process for preparing ultrafine silicalite particles having a diameter from about 0.1 micrometers to about 0.5 micrometers. The present invention also relates to a process for preparing a thin-film silicalite-silicone composite membrane that can be utilized in a pervaporation process for recovery of volatile organic compounds such as butanol from aqueous solutions and fermentation broths wherein the composite membrane comprises silicalite particles having a diameter from about 0.1 micrometers to about 0.5 micrometers.
Recent concerns over polluting the environment and the limited supply of economically accessible crude oil have generated a renewed interest in replenishable energy supply technologies. Acetone butanol ethanol (ABE) fermentation is a biological process which produces a replenishable supply of organic fuels and solvents. The maximum concentration of total solvents (acetone, butanol, and ethanol) in an ABE fermentation broth is typically about 20 g/L. Recovery of these solvents in more concentrated form by simple distillation, however, is economically unfavorable as compared to the production of conventional petroleum based products.
One technique that has recently seen advances such that it may be economically possible to recover organics from fermentation broths is pervaporation. Pervaporation is a separation technique wherein liquid volatile organic compounds such as butanol are preferentially transported across a thin membrane film. The feed side of the membrane is contacted by an aqueous liquid or fermentation broth containing the volatile organic compound to be recovered, while a vacuum or sweep gas is applied on the permeate side of the membrane as a driving force to selectively transport components of the liquid or broth through the membrane. The volatile organic compounds are collected on the permeate side of the membrane by condensation in a cold trap. Two characteristics of the membrane utilized in a pervaporation process define the membrane""s effectiveness in the separation of volatile organic compounds such as butanol in a pervaporation process: selectivity toward the desired species to be separated and flux or flow rate of the permeate through the membrane. Flux is generally defined as the rate at which permeate passes through the membrane and is generally reported in g/m2h.
To date, several types of membranes have been utilized in pervaporation processes with varying degrees of success. These include silicone rubber type membranes, polypropylene membranes, polytetrafluoroethylene membranes, liquid membranes, and poly[1-(trimethylsilyl)-1-propyne] membranes and other modified polymer-type membranes. Although these membranes have suitable properties for recovering dilute alcohols from aqueous solutions or fermentation broths, their selectivities are typically too low for most commercial applications.
In an attempt to increase the selectivity and flux of silicone rubber-type membranes, which typically have the most advantageous properties for pervaporation as compared to other membranes, silicalite particles have been introduced as a filler into silicone type membranes to create composite membranes. Silicalite particles, which generally have a diameter from about 1 micrometer to about 45 micrometers, are hydrophobic and are capable of selectively adsorbing organic solvents such as alcohols and acetone from aqueous solutions or fermentation broths. Organics such as butanol are adsorbed into the silicalite particle rendering the membrane highly impermeable to water molecules which greatly increases selectivity for organic solvents. Composite type membranes utilizing silicalite particles have an increased selectivity and good flux rate as compared to previously utilized membranes.
In order to further improve the selectivity of silicone rubber-silicalite composite membranes while maintaining a sufficient flux rate, attempts have been made to synthesize silicalite particles having a diameter suitable for introduction into a silicone membrane. Utilizing a hydrothermal gel method of silicalite preparation, Meng-Dong Jia et al. (Journal of Membrane Science, 73 (1992) 119-128) purport to have produced silicalite particles having a diameter of 0.3 micrometers to 0.4 micrometers. However, the preparation methodology disclosed by Jia et al. does not produce silicalite particles having diameters within this range.
Ravishankar et al. (Physiochemical Characterization of Silicalite-1 Nanophase Material, Journal of Physical Chemistry B, 1998, 102 (2633-2639)), Schoeman, et al. (Analysis of the Crystal Growth Mechanism of TPA-Silicalite-1, Zeolite, 1994, Vol. 14,September/October pp. 568), and Persson et al. (The Synthesis of Discrete Colloid Particles of TPA-Silicalite-1, Zeolite, 1994 Vol 14, September/October pp. 557) have synthesized silicalite particulates in sizes ranging from about 0.02 micrometers to 0.1 micrometers in diameter utilizing a clear solution silicalite synthesis method. Although a clear solution method is capable of preparing nano-sized silicalite particulates, large scale commercial exploitation of clear solution methods is not cost effective as such methods can take several days to produce acceptable product. Apart from throughput limitations, such processes require the use of numerous expensive super-centrifugation separation steps and equipment capable of producing revolutions per minute in excess of 12,000 in order to separate the nano-sized silicalite particles from the solution. Also, nano-sized particles produced by clear solution processes tend to aggregate into larger particulates during separation, drying, and calcining making it difficult to disperse the particulates in a membrane cast solution and produce composite membranes incorporating nano-sized silicalite particles having a uniform active layer.
Therefore, a need persists for an economically practical process that can produce silicalite particles having a small diameter suitable for incorporation into a pervaporation membrane exhibiting high selectivity toward organic solvents. A need also continues to exist for thin film silicalite composite membranes having high flux rates toward organic compounds such as alcohols.
Among the objects of the present invention, therefore, are the provision of a process for producing silicalite particles having a diameter from about 0.1 to about 0.5 micrometers; the provision of a process for producing a silicalite-silicone composite membrane containing a filler material comprised of silicalite particles having a diameter from about 0.1 micrometer to about 0.5 micrometers; the provision of a process for producing a silicalite-silicone membrane having a uniform active layer comprising silicalite particles have a diameter from about 0.1 micrometer to about 0.2 micrometers; and the provision of a process for preparing a silicone composite membrane which can be utilized in a pervaporation process for recovering volatile organic compounds
Briefly, therefore, the present invention is directed to a process for preparing silicalite particles. The process comprises mixing an alkylonium compound, a silica source, a base and water to form a gel which reacts to form silicalite particles having a diameter of from about 0.1 micrometers to about 0.5 micrometers. After the reaction, the silicalite particles are diluted with a liquid to form a liquid product mixture and the silicalite particles in the mixture are dispersed with ultrasonic waves. After the dispersion is complete, the silicalite particles are recovered from the liquid mixture and dried.
The invention is further directed to a process for preparing silicalite particles. The process comprises mixing an alkylonium compound, silica source, a base and water in a molar ratio of about 1:2-10:1-4:20-100,respectively, to form a gel. The gel is introduced into a sealed container and aged. After aging, the gel is heated to react the silica and alkylonium compound to form silicalite particles having a diameter of from about 0.1 micrometers to about 0.5 micrometers. Finally, the silicalite particles are recovered from the reaction mixture and dried.
The invention is further directed to a process for preparing silicalite particles. The process comprises mixing an alkylonium compound, silica source, a base and water in a molar ratio of about 1:2-10:1-4:20-100,respectively, to form a gel. The gel is introduced into a sealed container and aged. After aging, the gel is heated to react the silica and alkylonium compound to form silicalite particles having a diameter of from about 0.1 micrometers to about 0.5 micrometers. After the reaction, the silicalite particles are diluted with water to form a liquid product mixture and the silicalite particles in the mixture are dispersed with ultrasonic waves. Finally, the silicalite particles are recovered from the liquid product mixture and dried.
The invention is further directed to a process for preparing a thin-film silicalite-silicone composite membrane wherein the silicalite particles have a diameter of from about 0.1 micrometers to about 0.5 micrometers. The process comprises mixing silicalite particles, silicone rubber and iso-octane to form a membrane precursor mixture. After the precursor mixture is formed, ultrasonic waves are transmitted through the precursor to disperse the silicalite particles, silicone rubber and iso-octane. After dispersion of the precursor mixture, a silicone curing agent is introduced into the dispersed precursor mixture and the new mixture subjected to ultrasonic waves to disperse the curing agent. Finally, the mixture is pre-polymerized to increase the viscosity of the mixture and casted onto a support structure.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.